Geothermal steam which is steam found trapped in the earth is a potentially valuable source of energy, particularly in view of the ever increasing need to decrease reliance on fossil fuels, such as obtained from imported oil, as an energy source. Geothermal steam, like any high pressure steam, may be used, for example, to drive a turbine and generate electricity. Unfortunately, geothermal steam almost always contains contaminants such as CO.sub.2, CH.sub.4, NH.sub.3 and, most troublesome, H.sub.2 S.
Obviously, H.sub.2 S is a problem when geothermal steam is used as a source of energy from the standpoint of contamination of the environment, but it is also a problem with respect to its corrosive effect on the power generating equipment. The H.sub.2 S is, therefore, ideally removed from the high pressure steam upstream of the power generating equipment. Many efforts are known in the art to have been made to accomplish that end.
U.S. Pat. No. 4,196,183 to Li discloses a process for removing H.sub.2 S from geothermal steam by adding free exygen to the steam and contacting the mixture with an activated carbon catalyst at a temperature above the saturation temperature of the steam. In Li's process the H.sub.2 S is oxidized to free sulfur which is adsorbed on the activated carbon and which periodically must be removed therefrom. Li explains in his text that the superheating is necessary "to prevent binding of active catalyst sites by moisture."
U.S. Pat. No. 3,972,988 to Urban discloses a two-stage process for obtaining free sulfur from an aqueous sulfide solution. In the first stage, the sulfide is converted to a polysulfide which is then mixed with steam and decomposed to form a mixture which might be considered analogous to geothermal steam in that it comprises vapor containing H.sub.2 O, H.sub.2 S and NH.sub.3. This vaporous mixture is then mixed with air and contacted with a metal phthalocyanine catalyst in a second stage at a pressure not greater than 20 psig to produce sulfur (which maximum pressure is stated to be the essence of the invention), and a solution of a relatively minor amount of thiosulfate compound which can be disposed of or recycled to maintain a thoroughly wetted support bed. The stated advantage to the low pressure of the second stage is that the oxidation is performed in a vaporous phase, whereas in the prior art the oxidation is performed in a liquid phase.
The process of the present invention, unlike Li, requires that the geothermal steam be contacted with the oxidation catalyst at a temperature no higher than the saturation temperature, and unlike Urban requires a reactor pressure of at least 30 psig. The process of the present invention has been found to be uniquely suitable for the processing of geothermal steam where the objective is to remove small amounts of hydrogen sulfide rather than produce free sulfur for subsequent sale or commercial use.